Quadrature Synthetic Aperture Beamforming Front-End for Miniaturized Ultrasound Imaging

A quadrature synthetic aperture front-end receiver for B-mode ultrasound imaging is presented. The receiver targets small-scale imaging applications such as capsule endoscopy and low-cost portable devices. System complexity, area, power consumption, and cost are minimized using synthetic aperture beamforming (SAB), whereby signals are processed in a sequential manner using only a single channel. SAB is combined with quadrature (I/Q) sampling, which further reduces the bandwidth and computational load. I/Q demodulation is carried out using a full custom analog front-end (AFE), which comprises a low-noise, variable gain preamplifier, followed by a passive mixer, programmable gain amplifier (PGA) and active lowpass filter. A novel preamplifier design is proposed, with quasi-exponential time-gain control and low noise (5.42 nV√Hz input-referred noise). Overall, the AFE consumes 7.8 mW (static power) and occupies 1.5 mm × 1.5 mm in AMS 0.35 μm CMOS. Real-time SAB is carried out using a Spartan-6 FPGA, which dynamically apodises and focuses the data by interpolating and applying complex phase rotations to the I/Q samples. For a frame rate of 7 Hz, the power consumption is 3.4 mW/channel across an aperture of 64 elements. B-mode images were obtained using a database of ultrasound signals (2.5 MHz center frequency) derived from a commercial ultrasound machine. The normalized root mean squared error between the quadrature SAB image and the RF reference image was 13%. Image quality/frame rate may be tuned by varying the degree of spatial compounding.